Craze-resistant polymodal linear block copolymers with terminal tapered blocks

ABSTRACT

Sequential charging of initiator and monomers in a multistage solution polymerization process involving conjugated diene and monovinylaromatic monomers wherein the final charge is a mixture of these monomers results in the production of resinous polymodal linear block copolymers each with a terminal tapered block. A typical 5-stage charge sequence is as follows: Stage 1: (Si); Stage 2: (Si); Stage 3: (B); Stage 4: (Si); Stage 5: (S+B) wherein (Si) is monovinylaromatic monomer and initiator; (B) is conjugated diene monomer and (S+B) is monovinylaromatic monomer and conjugated diene monomer. At each stage, polymerization is allowed to continue until no free monomer is present. The trimodal block copolymer products comprise high, medium and low molecular weight species designated, respectively, as S1-S2-B1-S3-B2-&gt;S4; S2-B1-S3-B2-&gt;S4; and S3-B2-&gt;S4 wherein S1, S2 and S3 represent, respectively, polyvinylaromatic blocks resulting from polymerization of the first, second and third charges of monovinylaromatic monomer (stages 1,2 and 4); B1 represents a polydiene block resulting from polymerization of the first charge of conjugated diene monomer (stage 3); and B2-&gt;S4 represents a terminal tapered copolymer block resulting from polymerization of the conjugated diene and monovinylaromatic monomers (stage 5). The subject copolymers can be fabricated into sheets and molded objects possessing little color, a high degree of transparency and excellent impact resistance. These fabricated products do not craze on being impacted.

FIELD OF THE INVENTION

The invention relates to polymodal linear block copolymers with terminaltapered blocks.

In another aspect, the invention relates to a method of preparingpolymodal linear block copolymers characterized by a terminal taperedblock which involves sequential charging of initiator and monomers tothe polymerization zone.

BACKGROUND OF THE INVENTION

Although substantially transparent block copolymers have been preparedby a variety of initiator and monomer charge sequences, the prior artmaterials such as described in U.S. Pat. No. 4,335,221 are not entirelysatisfactory in regard to transparency, craze-resistance and color.Articles fabricated from some of the available block copolymers arecharacterized by low impact strength, a tendency to craze on beingimpacted, a blue coloration and a haziness particularly in thin sheets.

The deficiencies of available block copolymers are evident in packagingapplications such as blister packs for bandages and syringes. Thesetransparent colorless packs are intended to display and protect thecontents and maintain a sterile atmosphere within the pack. Crazing ofthese packs due to squeezing during shipping is very undesirable becausethe crazed packs and their contents are discarded on the presumptionthat the sterile atmosphere within the pack has been contaminated by anair leak.

Thus, there is a need for block copolymers which possess the capacity toimpart craze-resistance as well as transparency and little color tomanufactured articles particularly in the packaging field.

INVENTION

Thus, it is an object of this invention to provide polymodal linearblock copolymers which can be fabricated into products possessingcraze-resistance, little color and a high degree of transparency.

Another object of this invention is to provide blister packagingpossessing craze-resistance and a high degree of transparency.

Other aspects, objects and the several advantages of the invention willbe apparent to those skilled in the art from the description and theappended claims.

The inventive copolymers can be prepared by sequentially chargingmonomers and initiator to the polymerization zone wherein at least oneconjugated diene monomer is copolymerized with at least onemonovinylaromatic (monovinylarene) monomer in a 5-stage charge sequencecomprising:

Stage 1: (S_(i)): Initiator and monovinylaromatic monomer

Stage 2: (S_(i)): Initiator and monovinylaromatic monomer

Stage 3: (B): Conjugated diene monomer

Stage 4: (S_(i)): Initiator and monovinylaromatic monomer

Stage 5: (B and S): Conjugated diene monomer and monovinylaromaticmonomer

At each stage, polymerization is allowed to continue until essentiallyno free monomer is present. The terminal tapered block is produced inthe fifth stage by charging both monomers to the polymerization zone.

The instant polymodal linear block copolymers comprise high, medium andlow molecular weight species designated, respectively, as S₁ -S₂ -B₁ -S₃-B₂ →S₄ ; S₂ -B₁ -S₃ -B₂ →S₄ and S₃ -B₂ →S₄ wherein S₁, S₂ and S₃represent, respectively, polyvinylaromatic blocks resulting frompolymerization of the first, second and third charges ofmonovinylaromatic monomer (stages 1, 2 and 4); B₁ represents a polydieneblock resulting from polymerization of the first charge of conjugateddiene monomer (stage 3); and B₂ →S₄ represents a terminal taperedcopolymer block resulting from the polymerization of the final charge(stage 5) of conjugated diene monomer and monovinylaromatic monomer.

The instant copolymers contain about 55 to 95 preferably 70 to 80 weightpercent of monovinylaromatic monomer units and 45 to 5 preferably 30 to20 weight percent conjugated diene monomer units based on total weightof monomers charged to the polymerization zone.

Broad and preferred ranges for practicing the instant process aresummarized in Table A. The symbols L, S and B represent, respectively,organomonoalkali metal initiator such as n-butyllithium,monovinylaromatic monomer such as styrene and conjugated diene monomersuch as 1,3-butadiene.

                  TABLE A                                                         ______________________________________                                        Monomer and Initiator Ranges for Producing                                    Polymodal Copolymers                                                                                               Preferred                                Stage #        Component  Broad Range                                                                              Range                                    ______________________________________                                        1     (S.sub.i)                                                                              L-1 (phm).sup.1,2                                                                        0.02-0.04  0.02-0.03                                               L-1 (mhm).sup.3,4                                                                        0.312-0.625                                                                              0.312-0.625                                             S-1 (phm)  30-40      30-40                                    2     (S.sub.i)                                                                              L-2 (phm)  0.02-0.04  0.02-0.03                                               L-2 (mhm)  0.312-0.625                                                                              0.312-0.469                                             S-2 (phm)  10-20      10-15                                    3     (B)      B-1 (phm)   15-2.5    10-4                                     4     (S.sub.i)                                                                              L-3 (phm)  0.02-0.12  0.04-0.09                                               L-3 (mhm)  0.625-1.875                                                                              0.625 1.563                                             S-3 (phm)  10-20      20-10                                    5     (B and S)                                                                              (B-2) (phm)                                                                               30-2.5    20-16                                                   (S-4) (phm)                                                                               5-15      10-15                                    Totals         S (phm)    55-95      70-80                                                   B (phm)    45-5       30-20                                                   L (phm)    0.06-0.20  0.08-0.15                                               L (mhm)     1.25-3.125                                                                              1.250-2.657                              ______________________________________                                         # (S.sub.i) represents initiator and monovinylaromatic monomer; (B)           represents conjugated diene monomer; (B and S) represents                     monovinylaromatic monomer and conjugated diene monomer: the conjugated        diene was charged first followed immediately by the monovinylaromatic         monomer.                                                                      *phm is based on total monomers charged.                                      .sup.1 phm represents parts by weight per 100 parts by weight of total        monomers.                                                                     .sup.2 phm for L is based on nbutyllithium runs.                              .sup.3 mhm represents grammillimoles of organomonoalkali metal initiator      per 100 grams of total monomers.                                              .sup.4 mhm for L is applicable for any organomonoalkali metal initiators      based on the assumption that no poisons are present.                     

MONOMERS

The conjugated diene monomers contain 4 to 6 carbon atoms and include1,3-butadiene, isoprene, 2-ethyl-1,3-butadiene;2,3-dimethyl-1,3-butadiene and piperylene and mixtures thereof.Presently preferred is 1,3-butadiene.

The monovinylaromatic monomers contain 8 to 12 carbon atoms and includestyrene, alpha-methylstyrene, p-vinyltoluene, m-vinyltoluene,o-vinyltoluene, 4-ethylstyrene, 3-ethylstyrene, 2-ethylstyrene,4-tertbutylstyrene and 2,4-dimethylstyrene and mixtures thereof.Presently preferred is styrene.

POLYMERIZATION

The polymerization process is carried out in a hydrocarbon diluent atany suitable temperature in the range of -10° to 150° C. preferably inthe range of 0° to 110° C. at pressures sufficient to maintain thereaction mixture substantially in the liquid phase. Preferredhydrocarbon diluents include linear and cycloparaffins such as pentane,hexane, octane, cyclohexane and mixtures thereof. Presently preferred iscyclohexane.

Small amounts of polar organic compounds such as tetrahydrofuran arerequired in the hydrocarbon diluent to improve the effectiveness ofn-alkylmonoalkali metal initiators such as n-butyllithium. Amounts oftetrahydrofuran to provide from about 0.01 to 1.0 phm (parts per hundredparts of total monomer), preferably 0.02 to 0.1 phm are suitable.

The initiators can be any of the organomonoalkali metal compounds of theformula RM wherein R is an alkyl, cycloalkyl or aryl carbanioncontaining 4 to 8 carbon atoms and M is an alkali metal cation. Thepresently preferred initiator is n-butyllithium. Suitable amounts oforganomonoalkali metal initiator are encompassed by the ranges given inTable A.

The polymerization is carried out in the substantial absence of oxygenand water preferably under an inert gas atmosphere. Prior to thetermination treatment, the reaction mass contains a very high percentageof living molecules in which an alkali metal cation is positioned at oneend of each polymer chain. Impurities in the feed such as water oralcohol reduce the amount of monoalkali metal living polymer in thereaction mass.

At the conclusion of the polymerization process, the system is treatedas is known in the art with an active hydrogen compound such as water,alcohol, phenols or linear saturated aliphatic mono- and dicarboxylicacids to convert terminal carbon-lithium bonds on the living polymermolecules to carbon-hydrogen bonds resulting in the separation ofcopolymer. Preferably, the polymer cement, i.e., the polymer in thepolymerization solvent, is treated with terminating agents (water andcarbon dioxide) and then antioxidants before the solvent is flashed offto increase the solids content.

The resinous copolymers can be and usually are compounded with variousadditives such as release agents and the like before fabrication intouseful articles such as molded objects and sheeting.

The following Examples illustrate the invention.

EXAMPLE I

This example describes the preparation of inventive polymodal linearblock copolymers with one terminal tapered block. Styrene and1,3-butadiene were copolymerized in a 5-stage process usingn-butyl-lithium initiator. The runs were carried out under nitrogen in atwo-gallon jacketed stainless steel reactor. The anhydrous reactionmixtures were stirred continuously during the polymerization process.Termination was effected by the conventional use of carbon dioxide andwater. A mixture of Irganox 1076 (0.25 phm) andtris(nonylphenyl)phosphite (TNPP) (1.0 phm) was added prior to isolationof copolymer product. The inventive procedure is summarized in Table I.

                                      TABLE I                                     __________________________________________________________________________    Polymodal Linear Block Copolymers with Terminal Tapered Blocks                Run 1*                          Run 2#                                                               Polymerization                  Polymerization         Stage.sup.a                                                                         Components.sup.b                                                                      Gm.  phm.sup.c                                                                         Time (Min.)                                                                            Stage Components                                                                           Gm.   phm Time                   __________________________________________________________________________                                                           (Min.)                 1(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                      2134 156 15       1(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                     2134  157 14                           THF     0.68 0.05               THF    0.68  0.05                             S       445  32                 S      435   32                               NBL     0.34 0.025              NBL    0.408 0.03                       2(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                      182  13.2                                                                              16       2(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                     182   13.4                                                                              16                           NBL     0.34 0.025              NBL    0.34  0.025                            S       177  13                 S      177   13                         3(B)  C.sub.6 H.sub.12                                                                      91   6.6 15       3(B)  C.sub.6 H.sub.12                                                                     91    6.7 15                           B       136  10                 B      82    6                          4(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                      91   6.6 16       4(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                     182   13.4                                                                              17                           NBL     0.408                                                                              0.03               NBL    0.408 0.03                             S       231  17                 S      204   15                         5(B and S)                                                                          C.sub.6 H.sub.12                                                                      182  13.2                                                                              21       5(B and S)                                                                          C.sub.6 H.sub.12                                                                     182   13.4                                                                              17                           B       177  13                 B      258   19                               S       204  15                 S      204   15                         __________________________________________________________________________     .sup.a The symbols S.sub.i, B and S represent, respectively, styrene and      nbutyllithium initiator; 1,3butadiene; and styrene.                           .sup.b The symbols C.sub.6 H.sub.12, THF and NBL represent, respectively,     cyclohexane, tetrahydrofuran and nbutyllithium.                               .sup.c The symbol phm represents parts by weight of the designated            component per 100 parts by weight of total monomers. In run 1, the total      monomers charged amounted to 1370 grams. In run 2, the total monomers         charged amounted to 1360 grams.                                               *The butadiene/styrene weight ratio in this run was 23/77.                     # The butadiene/styrene weight ratio in this run was 25/75.             

Referring to the two invention runs in Table I, it is evident that theinvention products were prepared in a 5-stage process involving threeportions of styrene and initiator (stages 1, 2 and 4), one portion of1,3-butadiene (stage 3) and a final portion of styrene and 1,3-butadiene(stage 5). This sequence of charging reactants to the polymerizationzone results in a terminal tapered block on each high, medium and lowmolecular weight copolymer in the product.

As a result of the sequential charging of initiator in stages 1, 2 and 4(i.e., 3 charges of initiator), the copolymer products obtained in runs1 and 2 of Table I are referred to as polymodal, specifically trimodal.This term connotes the production of three molecular weight species:high, medium and low, arising by the initiation of new polymer chains instages 1, 2 and 4. Thus, in stage 1, living polystyrene chains areinitiated by n-butyllithium and propagation continues until no styrenemonomer is present resulting ideally in S₁ -Li living polymer chains ofcomparable molecular weight. The further addition of initiator andstyrene (stage 2) results in the initiation of new polystyrene chainsand propagation of the living S₁ -Li chains initiated in stage 1. Aftertwo stages wherein all styrene monomer has been polymerized, the bimodalspecies can be represented as:

    S.sub.1 -S.sub.2 -Li and S.sub.2 -Li

wherein S denotes polystyrene blocks and subscripts indicate the stageof monomer addition. The addition of 1,3-butadiene at stage 3 results inthe incorporation of the conjugated diene into each of the above livingpolymers to give bimodal copolymers:

    S.sub.1 -S.sub.2 -B.sub.1 -Li and S.sub.2 -B.sub.1 -Li

wherein S₁ and S₂ are polystyrene blocks and B₁ is a polybutadieneblock.

In stage 4 additional styrene and n-butyllithium are introducedresulting in the initiation of new polystyrene chains and propagation ofthe existing S₁ -S₂ -B₁ -Li and S₂ -B₁ -Li living polymers. After stage4 wherein all monomer has been polymerized, the trimodal species can berepresented as:

    S.sub.1 -S.sub.2 -B.sub.1 -S.sub.3 -L; S.sub.2 -B.sub.1 -S.sub.3 -Li and S.sub.3 -Li

designating high, medium and low molecular weight species. In stage 5 noadditional initiator is charged so that the term trimodal is stilldescriptive of the copolymers produced.

In stage 5 both styrene and 1,3-butadiene are introduced. Preferably,the monomers are charged as a mixture. Alternatively, the 1,3-butadieneis momentarily charged before the styrene. The rate of polymerization of1,3-butadiene is significantly greater than that of styrene so1,3-butadiene is incorporated more rapidly leading to living copolymerswith terminal blocks enriched in units from the diene. As the free1,3-butadiene monomer concentration decreases, the living copolymersbegin to incorporate both 1,3-butadiene and styrene monomers until apoint is reached at which all the 1,3-butadiene has been incorporatedand polymerization continues until all the styrene monomer has beenincorporated. This results in a terminal tapered block enriched withstyrene units at its terminal end preceded by a mixed zone of unitsderived from styrene and 1,3-butadiene in turn preceded by a blockenriched in units derived from 1,3-butadiene. After stage 5 the high,medium and low molecular weight trimodal species are:

    S.sub.1 -S.sub.2 -B.sub.1 -S.sub.3 -B.sub.2 →S.sub.4 -Li; S.sub.2 -B.sub.1 -S.sub.3 -B.sub.2 →S.sub.4 -Li and S.sub.3 -B.sub.2 →S.sub.4 -Li

wherein S₁, S₂ and S₃ represent, respectively, polystyrene blocksresulting from polymerization of the first, second and third charges ofstyrene (stages 1, 2 and 4); B₁ represents a polybutadiene blockresulting from polymerization of the first charge of 1,3-butadiene(stage 3) and B₂ →S₄ represents a terminal tapered copolymer blockresulting from polymer propagation in the presence of both 1,3-butadieneand styrene. After termination of the polymerization and productrecovery, these species can be represented as:

    S.sub.1 -S.sub.2 -B.sub.1 -S.sub.3 -B.sub.2 →S.sub.4 ; S.sub.2 -B.sub.1 -S.sub.3 -B.sub.2 →S.sub.4 and S.sub.3 -B.sub.2 →S.sub.4.

EXAMPLE II

This example describes the preparation of linear block copolymers withone terminal tapered block in accordance with the prior art teaching ofU.S. Pat. No. 4,335,221. Styrene and 1,3-butadiene were copolymerized ina 3-stage process using n-butyllithium initiator. The runs were carriedout under nitrogen in a two-gallon jacketed stainless steel reactor. Theanhydrous reaction mixtures were stirred continuously during thepolymerization process. Termination was effected by the conventional useof carbon dioxide and water. The procedure is summarized in Table II.

                                      TABLE II                                    __________________________________________________________________________    Control Runs for Preparing Linear Block Copolymers with Terminal Tapered      Blocks                                                                        Control Run*                    Control Run 2#                                                       Polymerization                  Polymerization         Stage.sup.a                                                                         Components.sup.b                                                                      Gm.  phm.sup.c                                                                         Time (Min.)                                                                            Stage Components                                                                           Gm.   phm Time                   __________________________________________________________________________                                                           (Min.)                 1(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                      2407 177 12       1(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                     2407  177 11                           THF     0.54 0.04               THF    0.54  0.04                             S       571  42                 S      571   42                               NBL     0.34 0.025              NBL    0.34  0.025                      2(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                      182  13.4                                                                              18       2(S.sub.i)                                                                          C.sub.6 H.sub.12                                                                     182   13.4                                                                              14                           NBL     0.95 0.07               NBL    0.95  0.07                             S       218  16                 S      218   16                         3(B and S)                                                                          C.sub.6 H.sub.12                                                                      91   6.7 20       3(B and S)                                                                          C.sub.6 H.sub.12                                                                     91    6.7 16                           B       340  25                 B      340   25                               S       231  17                 S      231   17                         __________________________________________________________________________     *The butadiene/styrene weight ratio in these runs (carried out in             accordance with the teaching of U.S. 4,335,221) was 25/75.                    .sup.a The symbols S.sub.i, B and S represent, respectively, styrene and      nbutyllithium initiator; 1,3butadiene; and styrene.                           .sup.b The symbols C.sub.6 H.sub.12, THF and NBL represent, respectively,     cyclohexane, tetrahydrofuran and nbutyllithium.                               .sup.c The symbol phm represents parts by weight of the designated            components per 100 parts by weight of total monomers. The total monomers      charged amounted to 1360 grams.                                          

Referring to the control runs in Table II, it is evident that theproducts were prepared in a 3-stage process involving two portions ofstyrene and initiator (stages 1 and 2) followed by a final portion ofstyrene and 1,3-butadiene (stage 3). This sequence of charging reactantsto the polymerization zone results in a terminal tapered block on eachhigh and low molecular weight copolymer in the product.

As a result of the sequential charging of initiator in stages 1 and 2(i.e., 2 charges of initiator,) the copolymer products obtained in runs1 and 2 of Table II are referred to as polymodal, specifically bimodal,in contrast to the trimodal inventive copolymers of Example I. The termbimodal connotes the production of two molecular weight species: highand low arising by the initiation of new polymer chains in stages 1 and2. Thus, in stage 1, living polystyrene chains are initiated byn-butyllithium and propagation continues until no styrene monomer ispresent resulting ideally in S₁ -Li living polymer chains of comparablemolecular weight. The further addition of initiator and styrene (stage2) results in the initiation of new polystyrene chains and propagationof the living S₁ -Li chains initiated in stage 1. After two stageswherein all styrene monomer has been polymerized, the bimodal speciescan be represented as:

    S.sub.1 -S.sub.2 -Li and S.sub.2 -Li

wherein S denotes polystyrene blocks and subscripts indicate the stageof monomer addition. The addition of 1,3-butadiene and styrene in stage3 with no additional initiator does not change the modality of theproducts.

In stage 3 both styrene and 1,3-butadiene are introduced to causeformation of tapered blocks. Preferably, the monomers are charged as amixture. The rate of polymerization of 1,3-butadiene is significantlygreater than that of styrene so 1,3-butadiene is incorporated morerapidly leading to living copolymers with terminal blocks enriched inunits from the 1,3-butadiene. As the free 1,3-butadiene monomerconcentration decreases, the living copolymers begin to incorporate both1,3-butadiene and styrene monomers until a point is reached at which allthe 1,3-butadiene has been incorporated and polymerization continuesuntil all the styrene monomer has been incorporated. This results in atapered 1,3-butadiene/styrene block enriched with styrene units at itsterminal end preceded by a mixed zone of units derived from styrene and1,3-butadiene in turn preceded by a block enriched with units derivedfrom 1,3-butadiene. After stage 3 the high and low molecular weightbimodal species are:

    S.sub.1 -S.sub.2 -B.sub.1 →S.sub.3 -Li and S.sub.2 -B.sub.1 →S.sub.3 -Li

wherein S₁ and S₂ represent, respectively, polystyrene blocks resultingfrom polymerization of the first and second charges of styrene (stages 1and 2) and B₁ →S₃ represents a terminal tapered copolymer blockresulting from polymer propagation in the presence of both 1,3-butadieneand styrene. After termination of the polymerization and productrecovery, the high and low molecular weight bimodal species can berepresented as:

    S.sub.1 -S.sub.2 -B.sub.1 →S.sub.3 and S.sub.2 -B.sub.1 →S.sub.3 .

The copolymer species present in the Example I products (inventive) andthe copolymer species present in the Example II products (prior artcopolymers) can be compared as below:

    ______________________________________                                        Inventive Copolymers                                                                            Prior Art Copolymers                                        (Trimodal)        (Bimodal; U.S. 4,335,221)                                   ______________________________________                                        High    S.sub.1 -S.sub.2 -B.sub.1 -S.sub.3 -B.sub.2 →S.sub.4                                 High Mol. Wt.                                                                             S.sub.1 -S.sub.2 -B.sub.1 →S.sub.                                      3                                           Mol. Wt.                                                                      Medium  S.sub.2 -B.sub.1 -S.sub.3 -B.sub.2 →S.sub.4                    Mol. Wt.                                                                      Low     S.sub.3 -B.sub.2 →S.sub.4                                                            Low Mol. Wt.                                                                              S.sub.2 -B.sub.1 →S.sub.3            Mol. Wt.                                                                      ______________________________________                                    

EXAMPLE III

This example compares the properties exhibited by test samples ofinvention copolymers (trimodal) with the properties of test samplesprepared from the control copolymers (bimodal). The invention copolymerswere prepared by the procedure summarized in Table I of Example I. Thecontrol copolymers were prepared by the procedure summarized in Table IIof Example II. The comparative results are compiled in Table III.

                  TABLE III                                                       ______________________________________                                        Comparison of Invention Copolymers and Control Copolymers                                  Control Polymers*                                                                           Invention Polymers                                              (Runs 1 and 2 (Runs 1 and 2                                      Property     of Example II)                                                                              of Example I)                                      ______________________________________                                        Melt Flow    13.3     6.1      7.2    10.4                                    (200° C., 5 Kg Wt)                                                     Haze, %       1.0     2.9      1.1    1.1                                     (50 mil specimen)                                                             Hunter (-) "b" value                                                                       14.7     24.7     4.6    6.0                                     Flex. Mod., psi × 10.sup.3                                                           163      151      181    172                                     Tensile Yield, psi                                                                         3635     3360     4900   4190                                    Hardness, Shore D                                                                           65       67       71     67                                     Vicat Temp. °F.                                                                     174      183      170    174                                     Elongation % 264      219      326    373                                     Gardner Impact, in-lbs                                                                     104       88      104    100                                     M.sub.W × 10.sup.-3                                                                  122      158      135    112                                     M.sub.N × 10.sup.-3                                                                   86       89      100     90                                     HI            1.42     1.75     1.35   1.25                                   M.sub.W Distribution                                                                       Bimodal  Bimodal  Trimodal                                                                             Trimodal                                Craze Resistant                                                                            No       No       Yes    Yes                                     ______________________________________                                         *Control copolymers were prepared in accordance with the teaching of U.S.     4,335,221.                                                               

Referring to the summary in Table III, it is evident that the trimodalinvention copolymers were clearly superior to the bimodal controlcopolymers in regard to blueness in that the Hunter (-) "b" values arelower for the instant copolymers indicating a very slight blue tint(values 4.6 and 6.0) compared to the distinct blueness in the controlcopolymers (values 14.7 and 24.7). In regard to % Haze and GardnerImpact values, the invention copolymers and control copolymers werecomparable; however, the bimodal control copolymers crazed on beingimpacted whereas the instant trimodal copolymers did not craze on beingimpacted.

Reasonable variations and modifications of the present invention whichwill become apparent to those skilled in the art can be made in thisinvention without departing from the spirit and scope thereof.

That which is claimed is:
 1. A sequential mode of block copolymerizationcomprising:polymerizing under solution polymerization conditions atleast one monovinylaromatic monomer containing 8 to 12 carbon atoms andat least one conjugated diene monomer containing 4 to 6 carbon atoms ina ratio of about 55 to 95 weight percent monovinylaromatic monomer and45 to 5 weight percent conjugated diene monomer, by a sequential chargepolymerization process wherein at least two separate charges consistingof said monovinylaromatic monomer and an organomonoalkali metalinitiator precede a separate charge of conjugated diene monomer followedby an additional separate charge of said monovinylaromatic monomer andsaid initiator before a final charge of said monovinylaromatic monomerand said conjugated diene monomer; each monomer charge polymerizes tosubstantial completion prior to addition of any subsequent charge;wherein said sequential charge polymerization process employs a 5-stagecharge sequence:Stage 1: (S_(i)) Stage 2: (S_(i)) Stage 3: (B) Stage 4:(S_(i)) Stage 5: (B and S) wherein (S_(i)) represents organomonoalkalimetal initiator and monovinylaromatic monomer, (B) represents conjugateddiene monomer and (B and S) represents conjugated diene monomer andmonovinylaromatic monomer thereby producing polymodal, craze-resistant,low color, transparent linear resinous copolymers each with a terminaltapered copolymer block of said monomers.
 2. The process of claim 1employing ranges of monomer additions and of organomonoalkali metalinitiator added at each stage:

    ______________________________________                                        Stage 1:      L-1 (mhm) 0.312-0.625                                                         S-1 (phm) 30-40                                                 Stage 2:      L-2 (mhm) 0.312-0.625                                                         S-2 (phm) 10-20                                                 Stage 3:      B-1 (phm)  15-2.5                                               Stage 4:      L-3 (mhm) 0.625-1.875                                                         S-3 (phm) 10-20                                                 Stage 5:      B-2 (phm)  30-2.5                                                             S-4 (phm)  5-15                                                 Totals:       S (phm)   55-95                                                               B (phm)   45-5                                                                L (mhm)    1.25-3.125                                           ______________________________________                                    

wherein L represents organomonoalkali metal initiator, B representsconjugated diene monomer and S represents monovinylaromatic monomer. 3.The process of claim 2 employing ranges of monomer additions and oforganomonoalkali metal initiator added at each stage:

    ______________________________________                                        Stage 1:      L-1 (mhm) 0.312-0.625                                                         S-1 (phm) 30-40                                                 Stage 2:      L-2 (mhm) 0.312-0.469                                                         S-2 (phm) 10-15                                                 Stage 3:      B-1 (phm) 10-4                                                  Stage 4:      L-3 (mhm) 0.625-1.563                                                         S-3 (phm) 20-10                                                 Stage 5:      B-2 (phm) 20-16                                                               S-4 (phm) 10-15                                                 Totals:       S (phm)   70-80                                                               B (phm)   30-20                                                               L (mhm)    1.25-2.657                                           ______________________________________                                    

wherein L represents organomonoalkali metal initiator, B representsconjugated diene monomer and S represents monovinylaromatic monomer. 4.The process of claim 1 employing 1,3-butadiene as said conjugated dienemonomer, styrene as said monovinylaromatic monomer and n-butyllithium assaid organomonoalkali metal initiator.
 5. Polymodal linear blockcopolymers produced in accordance with the process of claim
 1. 6.Polymodal linear block copolymers of 1,3-butadiene and styrene withterminal tapered blocks produced in accordance with the process of claim4.
 7. Blister packs comprising the polymodal linear block copolymerswith terminal tapered blocks produced in accordance with the process ofclaim
 1. 8. Blister packs comprising the polymodal linear blockcopolymers of 1,3-butadiene and styrene with terminal tapered blocksproduced in accordance with the process of claim
 4. 9. The process ofclaim 1 wherein in stage 1 the charge includes 0.01 to 1.0 phm oftetrahydrofuran based on the total amount of monomers charged.